Various color recording methods utilizing an electro-photographic method have been proposed. One of them is the repeated developing method. This method forms bi-value or tri-value electrostatic latent images on a photosensitive member, develops the first latent image by a first processor and then the second latent image by a second processor, and transfers the ultimately formed toner images to a copying paper at the same time to obtain a color picture.
This repeated developing method has a problem, however, in that the photosensitive member already carrying the toner image formed by the first developing process is rubbed again by the developer in the second developing process. As a result, the toner image formed by the first developing process is greatly disturbed in and after the the second process, leading to the extremely disturbed, i.e., smeared, final color picture. Therefore, it is very important in picture formation by the repeated developing method that the second state developing process be performed without disturbing the toner image formed at the first developing stage.
A developing method that would cause little disturbance of the toner image on the photosensitive member is conceivable which uses a single-ingredient developer for the second-stage developing process. In the single developer non-contact developing method, however, it is difficult to increase the developing speed and the method is not preferable in this respect to the magnetic brush developing method that uses the developer consisting of two components, i.e., a carrier and toner.
A two-ingredient developer, as is normally employed in the magnetic brush developing method, is composed of substances such as iron, ferrite, etc., which have a grain size of 80-120 microns. In the magnetic brush developing method using a two-ingredient developer, the developer is carried by a non-magnetic sleeve of a developing roll having magnet rolls therein and the latent image is developed by rubbing it with a magnetic brush. In order to obtain a sufficient developing density, it is necessary to set the ratio of the linear speed of the latent image carrier to the surface speed of the developing roll to be 1:3-4. This causes the problem that the toner image formed in the first developing process of the repeated developing method is rubbed and disturbed by the tip part of the magnetic brush in the second developing process.
To overcome these problems, Japanese Pat. Nos. 36889/1980 and 79970/1982 disclose methods wherein the rubbing force of the second magnetic brush device against the latent image carriers is made smaller than the rubbing force of the first magnetic brush device to reduce the toner scratch-off effect of the second magnetic brush. Namely, in the '889 patent, the surface movement linear speed of the developing roll of the second developing device is made equal to the surface speed of the latent image carrier. In the '970 patent, the magnetic flux density of the main pole magnet of the developing roll of the second developing device is made less than that of the first developing device to reduce the toner scratch-off effects. Further, Japanese Patent No. 126665/1985 proposes a color developing device, which uses a two-ingredient developer made of magnetic carriers with less than 50 micron grain size mixed with toner particles.
The device described in the '889 patent has the disadvantage that since the linear surface speed of the developing roll of the second image processor is equal to the surface speed of the latent image carrier, the second processor's developing ability is weakened and the developer concentration is lowered. The device described in the '970 patent has the drawback that the magnetic flux density of the main pole magnet of the developing roll of the second processor is too small (300-500 gauss) to develop images in a sufficiently high density. Further, in the device disclosed in the '665 patent, the use of the carriers with a smaller grain size has improved the image turbulence phenomenon, but a so-called "carry-over" phenomenon caused by the carriers moving from the processor to the photosensitive member surface becomes more prevalent as the carrier grain size becomes smaller. The magnetic force must be increased to avoid the carry-over phenomenon, and the carrier grain size must be increased to some extent to raise the magnetic force. Therefore, the control of the carrier particle diameter alone cannot produce satisfactory results.
Systems for developing latent color images have been embodied as color copier machines that reproduce pictures on a manuscript or other document by means of 3 kinds of color decomposing filters, such as, red (R), blue (B), and green (G) filters. The copier performs the reproduction process separately for each of these components of color images and superimposes the images on a copying paper to obtain an image of the original color picture.
In such a color reproduction machine or color copier, the precise registration of differently colored pictures on the copy paper is also necessary which complicates the construction and control system of the machine, enlarges its size, and increases its manufacturing cost. Moreover, multi-colored manuscripts or documents as are commonly duplicated are mostly composed of black letters, figures, and other markings with only a small percentage of underlines, marks, etc., being in red or other colors. Therefore the use of a full-color reproduction maching is extremely uneconomical for such documents. In a full-color copy machine, any black picture is expressed by the mixture of 3 colors, will not be purely black, and has lower reproducibility than if reproduced with a purely black toner. Further, letters included in the black picture part of a manuscript are mostly made of fine lines and become illegiblle if the registration is not precise.
Therefore, a bi-color copying machine using only black toner and a toner of another color would be more useful in reproducing many documents comprised mostly of black letters and pictures. It is desired that these black letters and pictures be equal in quality to those reproduced by a conventional monochrome copying machine. Further, in order that most documents composed of two colors may be reproduced, it is preferable that two or three different toner colors (red, blue, green, etc.) be individually selectable to be combined with black toner according to the circumstances.
Japanese Pat. Nos. 73063/1980, 36767/1986, 36768/1986, 48871/1986 and 162755/1981 disclose bi-color copying machines intended to meet the aforesaid requirements. The '063 patent describes a bi-color copying machine for forming electrostatic latent images of the letters and figures in black color and another specific color on a photosensitive member by using only an analog optical system. The analog optical system is a device that focuses photo images directly to the photosensitive member by means of mirrors and lenses. Such a device will hereinafter be called an "imaging optical system."
A copying machine of this type has the problem that a tri-value latent image having 3 levels of potential is actually formed, leading to the occurrence of hybrid colors at the latent image border by the fringe electric field and to the deterioration of black pictures compared with those reproduced by a conventional, monochrome copying machine. Further, the machine uses a special laminated photosensitive member capable of forming the tri-value latent image and has a complicated electrostatic latent image forming process, which increase its production cost.
The device disclosed in the '768 patent is intented to overcome these problems, and exposes two photosensitive members using an imaging optical system. One of optical systems is an ordinary photosensitive member and the other optical system is a laminated photosensitive member for forming the latent image corresponding to a specific color. The electric latent images on the first and second photosensitive members are developed by black toner and the toner of another specific color, respectively, and transferred one after the other onto a copying paper.
This method reduces the problem of deteriorated black pictures but has the inherent problem that since two differently colored images are transferred to a copying paper one after the other by two distinct operations, the machine must operate to register precisely the two images. This also leads to a higher cost. Also, the machine must be enlarged to accommodate both kinds of photosensitive members, and the use of a laminated photosensitive member complicates the reproduction process, and increases cost. Also, in the method to separate colors by a laminated photosensitive member, the colors to be separated are fixed according to the characteristics of the member, so it is impossible to increase the number of specific colors.
The system described in the '755 patent copies the black picture on a copying paper by the electronic reproduction process using an imaging optical system, and then extracts the picture in a specific color by an image sensor and additionally transfers it to the copying paper by a recorder with the electrostatic recording system. This method is more advantageous in the separation and reproduction of the specific color than the above-mentioned method using two imaging optical system, but requires highly precise registration as the picture in a specific color has to be superimposed on the black picture already formed on the copying paper. This makes the machine expensive and it must be large enough to accommodate the two kinds of independent imaging devices.
The '871 patent discloses the technology which forms the electrostatic latent images in several different colors, each image being formed on a different photosensitive member by a different imaging optical system, and transfers them to a copying paper one after another. This involves a complicated transfer process and has the problem of registration.
The device shown in the '767 patent uses two photosensitive members, forms the electrostatic latent images of the manuscript or document pictures on the photosensitive members by means of an imaging optical system, erases unnecessary parts from each latent image by an eraser, develops these images by different color developers, and transfers them to a copying paper one after the other. This method also requires high-precision registration because the transfer of the two pictures in different colors is made at a different place.